In recent years, a doctor displays a captured medical image on a monitor and interprets the medical image on the monitor to make a diagnosis (image diagnosis) of a state or temporal change of a morbid portion of a patient.
As an apparatus (medical imaging apparatus) for generating a medical image of this type, a plain radiographic apparatus, X-ray computed tomography apparatus (X-ray CT), nuclear magnetic resonance imaging apparatus (MRI), nuclear medicine diagnosis apparatus (SPECT, PET, etc.), ultrasonic imaging apparatus (US), and the like are available.
These medical imaging apparatuses respectively capture images of interior portions of human bodies using different imaging means, and respectively have different features.
For example, the plain radiographic apparatus irradiates a human body with X-rays, and records transmitted X-rays, thus capturing an image by obtaining a parallel projection image of X-ray absorption ratios of the interior portion of a human body. This plain radiographic apparatus has an advantage of acquiring information of the interior portion of a human body to have a relatively low dose within a relatively short period of time. On the other hand, since information of the radiation direction of X-rays is lost, detailed information of the interior portion of a human body cannot be obtained.
In case of the X-ray CT, a human body is irradiated with X-rays from various directions to obtain a large number of fluoroscopic images, and generates three-dimensional information of the interior portion of a human body by analyzing these fluoroscopic images. For this reason, the X-ray CT has an advantage of obtaining detailed information in the interior portion of a human body, although the dose and imaging time are increased compared to the plain radiographic apparatus.
Since the MRI can obtain three-dimensional information of the interior portion of a human body as in the X-ray CT, and is configured to image information using a magnetic resonance phenomenon, it can obtain physical information different from that of the X-ray CT which images the X-ray absorption ratios. On the other hand, at the time of imaging by the MRI, a contrast agent has to be normally applied to a human body, resulting in a heavy work load at the time of imaging.
The ultrasonic imaging apparatus (US) irradiates a human body with ultrasonic waves, and detects the ultrasonic waves reflected from the interior portion of the human body to acquire information of the interior portion of the human body. Then, by using a method called “B mode”, three-dimensional information of the interior portion of the human body can be obtained. With this arrangement, unlike the X-ray CT and the like, there is no invasion such as exposure with respect to a human body, and real-time imaging can be attained. Hence, there is an advantage of making imaging and observation at the same time. On the other hand, an image (ultrasonic image) of the ultrasonic imaging apparatus has a high signal to noise ratio, which may often disturb diagnosis.
In image diagnosis which makes a diagnosis of a patient using medical images generated by the medical imaging apparatuses, an appropriate medical imaging apparatus is selected according to portions to be diagnosed and the types of diseases in consideration of the aforementioned different characteristics of the respective medical imaging apparatuses.
Furthermore, in order to realize more accurate image diagnosis, a plurality of medical images generated by a plurality of medical imaging apparatuses are used in some cases. For example, medical images of a single patient are captured using both the ultrasonic imaging apparatus (US) and MRI, and are combined to obtain more effective information in image diagnosis.
In order to combine a medical image (ultrasonic image) obtained from the ultrasonic imaging apparatus with a three-dimensional medical image obtained from a medical imaging apparatus other than the ultrasonic imaging apparatus, the ultrasonic image and three-dimensional medical image have to be registered.
However, in order to register the ultrasonic image and three-dimensional medical image, some problems have to be solved. First, the ultrasonic imaging apparatus normally captures an image while a document or operator holds an ultrasonic probe by the hand and freely moves it. For this reason, the capturing position of the captured ultrasonic image in a space with reference to a human body is not always revealed from only the ultrasonic image.
Second, the ultrasonic imaging apparatus normally captures a two-dimensional tomographic image of the interior portion of a human body, and the space dimension of information that can be acquired by imaging is different from that acquired by the X-ray CT, MRI, SPECT, PET, or the like, which generates a three-dimensional medical image.
As one approach to solve such problems, a method of measuring the position and orientation of the ultrasonic probe using an external sensor is available. More specifically, patent reference 1 discloses a technique in which a device for measuring the position and orientation is attached to the ultrasonic probe, a slice image corresponding to a portion captured by the ultrasonic imaging apparatus is acquired from a three-dimensional medical image captured in advance in accordance with the measurement value of the device, and two images are combined and displayed. According to this technique, an ultrasonic image as a two-dimensional tomogram and a three-dimensional medical image generated by another medical imaging apparatus can be observed in association with each other.
As another approach to solve the problems, it has been conventionally examined that an ultrasonic image and three-dimensional medical image are registered using image information included in the ultrasonic image and three-dimensional medical image. More specifically, non-patent reference 1 has proposed a method in which an ultrasonic simulation image generated based on a three-dimensional medical image acquired in advance and an ultrasonic image actually captured by the ultrasonic imaging apparatus are registered by associating them with each other based on image information.